Same problem for me !
Want to use a Radeon 8500 with nForce 420 but the stupid agp bus is not recognized by the generic agpgart module and so, if I want to use my linux with this Motherboard, I have to use the _very_ bad integrated graphic.
There is another solution : change the motherboard ! I was thinking about nForce2 but now, I'm sure to avoid this !
Nvida can avoid to make the users _very_ angry (Like me) by providing an agp driver in the nforce unifed driver ! Think about this !